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1. Technical Field
The present invention relates to automobile fuel level sensors, and in particular, to fuel sensors using Archimedes""s buoyant force principle.
2. Description of the Related Art
Conventional sensors for sensing the level of fuel in a motor vehicle fuel tank include a float which follows the level of the fuel tank due to the buoyant forces acting on the float. The float includes an arm coupled to a variable resistor or resistor card at a contact. As the float moves in accordance with the fuel level, the contact follows a path on the resistor card causing a variable resistance value. A fuel gauge electrically coupled to the resistor pad via a control module measures this variable resistance value and displays a corresponding fuel level.
The performance of float-type fuel sensors is largely effected by the size and configuration of the fuel tank. Fuel tanks in modem automobiles and other motor vehicles are becoming smaller as the engines become increasing more energy efficient. Moreover, in an effort to minimize overall space requirements of the vehicle, fuel tanks in current motor vehicles are configured as needed to fit within available spaces and around other components, such as trunks, tire wells and struts. The size and configuration constraints render float-type fuel sensors disadvantageous for a number of reasons.
First, the float arm must be sized as needed to extend from the variable resistor at one end to the float at the other, pivoting through a radius as the float follows the level of fuel between full and empty fuel levels. Thus, the float arm must be packagable in the fuel tank such that it can travel through a radius sweeping the height of the fuel. Generally, this means that the fuel tank cannot be narrower than the radius swept by the float arm. Consequently, this must be accounted for when designing the fuel tank, which may lead to less then ideal space utilization. Additionally, these fuel sensors are typically less accurate in low-profile fuel tanks wherein the floats travel a lesser path and the corresponding resistance values are less distinctive. This is particularly the case when the tank is full or near empty.
The irregular configuration of modern automobile fuel tanks also causes the volume of fuel added or consumed per change in fluid level height to be non-linear. That is, for example, a float-type fuel probe will inaccurately indicate as the same fuel volume a one inch change in the height of the fuel in two sections of the fuel tank having different cross-sectional areas. Consequently, the output resistance values of the fuel sensor must be specially processed to be calibrated for each fuel tank configuration.
Moreover, due to the mobile nature of motor vehicles, fuel within the fuel tanks is virtually in constant motion while driving. The float and float arm of control fuel sensor are typically repositioned by fuel slosh and inertial forces occurring while the vehicle is moving. For example, changing lanes, turning comers, driving over pot-holes and the like can cause the float and float arm to change the resistance value so that an erroneous fuel level measurement is output to the fuel gauge.
According, an improved fuel level sensor is needed that is particularly suited for use in current motor vehicle fuel tanks.
The present invention provides a fuel sensor that is suited for sensing the level of fuel in a fuel tank of a motor vehicle. Using Archimedes""s principle, buoyant forces acting on a substantially stationary probe are detected by a strain gauge giving resistance values calibrated to indicate the current fuel level in the tank. The probe is sized to fit in a wide variety of fuel tanks. It is lightweight such that it is highly sensitive to small variations in fuel level and substantially reduces errant fuel level measurements from inertial forces and fuel slosh caused by vehicle movement.
Specifically, the fuel sensor is affixed to an interior mounting within the fuel tank. The fuel sensor has a cantilever member extending in a substantially horizontal direction from the mounting to a free end. The cantilever member is substantially rigid but can be deflected in the vertical direction. A probe is affixed at the free end of the cantilever member and extends therefrom in a substantially vertical orientation nearly the entire height of the fuel tank, encountering a vertical buoyancy force equal to the volume of fuel it displaces. A stabilizer member is affixed at one end to the mounting and at an opposing end to the probe. The stabilizer member has a pivot segment between the ends permitting the probe to move vertically but substantially restricting movement of the probe in the horizontal plane. A strain gauge is rigidly coupled to a surface of the cantilever member effecting an electrical change corresponding to the strain in the cantilever member caused by the buoyancy force acting on the probe. This electrical change can be any measurable change in an electrical property, such as resistance, capacitance and inductance, and can be used to indicate the level of fuel in the fuel tank.
In one aspect of the fuel sensor of the present invention, the probe is low-cost and lightweight, preferably of a closed-cell foam, and defines a profile corresponding to the profile of the fuel tank substantially throughout the height of the fuel tank. The probe can be quickly removed from and connected to the cantilever member such that the fuel sensor can be made to include probes of various profiles as needed to use in various fuel tank configurations. Moreover, the low density of the probe makes it sensitive to small changes in fuel level.
In another aspect of the present, the stabilizer member and a fuel delivery module (FDM) cup act to reduce side loads on the probe from fuel sloshing within the fuel tank that can corrupt the fuel level measurement. The FDM cup substantially surrounds the probe and shields the probe from sloshing fuel. The stabilizer member connects the probe at a second end to the fuel tank mounting, and in so doing, provides opposing forces that negate the affect of the side loads on the a cantilever member such that little or no strain is sensed by the strain gauge. Thus, the fuel sensor of the present invention can provide accurate fuel level measurements by mitigating the effects of fuel sloshing.
In another aspect, the cantilever member can be a ceramic board onto which the strain gauge is etched. This integrally bonds the strain gauge to the cantilever member in a cost effective and permanent manner suitable for use in a fuel tank.
The present invention also includes a method for sensing the level of fuel in a motor vehicle fuel tank. The method includes displacing fuel with a sensor probe positioned in a substantially vertical orientation and extending from near a top to near a bottom of the fuel tank. A flexing member to which the probe is attached is deflected by a buoyant force of the fuel acting on the probe equal to the weight of the volume of fuel displaced by the probe. A strain gauge senses the strain in the flexing member resulting from the buoyant force. A voltage difference across the strain gauge is measured and calibrated to correspond to a particular fuel level. The corresponding fuel level is then displayed on a fuel gauge. The method can further include periodically sampling the voltage difference and averaging multiple voltage samples over a prescribed period.
The aforementioned and still other advantages of the present invention will be apparent from the description of the preferred embodiments which follow.